Product ebs R12.0.6
I am facing issue with event alert.the query is fetching the records but mails are not received by user. Some of the mails are received but some are not for the same user. Please explain the flow from triggering the alert to receiving the mail by recipient or provide some query to get the details from back end. I have checked the alert definition everything is fine in the setup. Please share query for wf_notification_out and wf_deffered table how we can query in this table based on the alert or the mail content in order to verify User are getting the mails from alert.

You can query wf_notifications table and check the values of status and mail_status columns.
The following are the values for mail_status column
MAIL - email yet to be sent
SENT - email is sent already
null - email cannot be sent